1. Field of the Invention
The present invention relates to a system for refining UMG Si using a steam plasma torch. More particularly, the present invention relates to a system for refining UMG Si which refines UMG Si into solar-grade Si using a steam plasma torch.
2. Description of Related Art
The global solar cell market has been growing strongly by annual average growth of over 35% since the mid-1990s wherein crystalline silicon-based solar cells account for more than 97% of solar cells in the market in respect of material and take the initiative in market growth. In the meantime, high purity silicon material which is a core material for solar cells has been supplied by scraps of single crystalline silicon for semiconductor wafers or off-specification polycrystalline silicon. However, the global solar cell market has been rapidly growing since the mid-1990s and the supply did not fall short of the fast growing demand with the entrance toward the industrialized phase and consequently the solar cell market has reached the situation of serious supply shortage since 2004. Shortage phenomenon of silicon material will be continued worldwide for some time due to the demand for investment in facilities on a large scale in order to manufacture the polycrystalline silicon raw material.
At the present time, 90% or more of the global production of high purity polysilicon is produced by a Siemens process, which is also called as trichlorosilane (TCS) method and developed by Siemens in Germany. In the Siemens process, the TCS refined to high purity is mixed, to increase a recovery rate, with hydrogen gas and introduced into in an extraction area at 1,000 to 1,100° C., followed by extraction of high purity solid polycrystal, thereby producing high purity polysilicon. The silicon produced by this process is high in cost and is so high pure that a content of impurities is at ppb level. However, Si for a solar cell does not require such high purity.
Therefore, nowadays, there have begun efforts of producing upgraded metallurgical grade (UMG) Si of about 5N purity (99.999%) from metal Si through an inexpensive process and using the produced UMG Si as the material for a solar cell. Use of the UMG Si has an advantage that it is possible to halve the ratio of raw material, which accounts for 30% of fabrication cost of crystalline Si solar cell. However, the UMG Si has higher concentration of boron and phosphorus that are used as dopant, as compared with that required for a substrate to be used as a solar cell.
Therefore, in order to use the UMG Si as a solar cell, it is essential to raise the purity thereof. The UMG Si may include other impurity elements such as boron, phosphorus, arsenic and antimony. According to types of the impurity element, different impurity refining methods are required.
There have been performed studies on a process for refining the UMG Si which allows obtaining of polycrystalline wafer and photocell capable of being competitive with classical material in aspects of both converting efficiency and cost.
Some study groups have been studied refinement of metallurgical grade Si using plasma. A study group in France has used argon plasma which is induced by adding oxygen as reactive gas. All experiments were performed at a small scale (<250 g). Also, other Japanese study group has been developed a refinement process in which electron beam, arc plasma and directional solidification are associated from the study on removal of boron, or one of the impurities. The electron beam was used to remove the phosphorus, the plasma treatment was used to remove the boron in association with use of water, and directional solidification consisting in two steps was used to remove metal impurities. An amount of Si processed in these studies has reached 300 kg per batch.
However, though it is possible to produce solar grade Si directly from the metallurgical grade Si, the aforementioned process has problems that refining effect is low and cost effectiveness is not high as the plasma process is performed at a low temperature and productivity is low as it is a batch type process. In addition, there exists a problem of recontamination due to impurities in a chamber during UMG Si refining process.